Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One

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Malaysian Journal of Chemistry, 2016, Vol. 18(1), 1–8

Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds Masoumeh Zakeri 1*, Mohamed Mahmoud Nasef 1,2, Ebrahim Abouzari-Lotf 1,2 and Hoda Haghi 3 1

Institute of Hydrogen Energy, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia 2 Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia 3 Department of Chemistry, School of Sciences, Alzahra University, Vanak, Tehran, Iran *Corresponding author (e-mail: [email protected])

Green and one-pot procedure was reported for the preparation of 1,1,3-triheteroaryl compounds from indoles and α,β-unsaturated carbonyl compounds using heteropoly acid as a reusable heterogeneous catalyst. The desired products were obtained in high yields (60%–80%) at EtOH/H2O media. Keggin heteropoly acid catalyst can be reused for at least four times without significant loss in the catalytic activity and change of chemical structure. The catalyst recyclability, simplicity, environmental friendliness, and convenient operation of the reaction suggest the potential of the procedure as a real alternative to conventional reaction protocol. Key words: Keggin heteropoly acid, H3PMo12O40, indole, α,β-unsaturated aldehyde Received: October 2014; Accepted: September 2015

Biologically active heterocycles, especially indoles have been reported to possess a wide variety of properties such as anticancer [1], cardiovascular [2] and antibacterial [3]. Furthermore, it was reported that the substitution of heterocyclic moiety at 3-position of the indole ring obviously influence the anti-inflammatory activity [4]. One of the simple and direct methods for the synthesis of 3-substituted indoles involves the conjugate addition of them to α,β-unsaturated carbonyl compounds or reaction of 2 or 3 equiv. of indoles with the carbonyl group in the presence of either Brønsted [5] or Lewis acids [6]. However, these one-pot methods make the use of high-priced catalysts such as AuCl3 [7] and Zr(OTf)4 [8], the dangerous catalyst such as SbCl3 [9], cerium ammonium nitrate (CAN) [10], and AlCl3 [11]. For this reason, cheaper and green acid catalysts with low toxicity are desired. The metal-oxygen clusters

specially Keggin-type anions have stimulated much attention over the last few decades due to the strong Brønsted acidity, multi-stage redox activity and remarkable thermal and hydrolytic stability [12]. Among them, the Keggin-type HPAs have long been known to be good catalysts for oxidation reactions [13] and heterogeneous acidic catalysis in green chemistry [14]. Their catalytic properties can be tuned by changing the identity of charge-compensating countercations, heteroatoms and framework metal atoms [15]. In continuation of our work on the synthesis of heterocyclic compounds [16] and catalytic properties of heteropoly acids in the synthesis of organic compounds [17–20], we in this paper studied a convenient and green method for the onepot preparation of 1,1,3-triheteroaryl compounds from indoles and α,β-unsaturated carbonyl

Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

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compounds using H3PMo12O40 (3 mol%) as a green, reusable and superior catalyst in a mixture of water and ethanol as a solvent. Experimental Material and Methods Melting points were measured by using the capillary tube method with an electro thermal 9200 apparatus. 1HNMR spectra were recorded on a Bruker AQS AVANCE-300MHz spectrometer using TMS as an internal standard (CDCl3 solution). IR spectra were recorded using KBr disk on the FT-IR Bruker Tensor 27. All products were well characterized by comparison with authentic samples by TLC, spectral and physical data. H3PMo12O40, Al-MCM-41, Mn(pbdo)2Cl2/MCM41, Mn(pbdo)2Cl2/SBA-15 and phenyl phosphonic acid/SiO2 were prepared in a classical way as described in the literature [21–26]. MgBr2. Et2O, NbCl5, KAl (SO4)2.12 H2O, MCM-41, Nano TiO2, Phenyl phosphonic acid and Montmorilonit K10 were purchased from Sigma-Aldrich. General procedure for synthesis of 1,1,3-triheteroaryl Compounds. H3PMo12O40 (3 mol%) and indole (5 mmol) were dissolved in 6.0 ml of EtOH/H2O (2/1) mixed solvent. Crotonaldehyde (1 mmol) was added dropwise into the reaction system by a syringe and the reaction mixture was stirred at room temperature. Progress of the reaction was monitored by TLC. After completion of the reaction, the solid product was collected by filtration, washed with cold water and aqueous ethanol and purified by silica gel column chromatography using petroleum etherethyl acetate (8:2) as eluent to give 1,1,3-tris(3indolyl)cyclohexane 3a.

3a: FT-IR (KBr): 3395, 2920, 1705, 1610, 1455, 1415, 1337, 1235 cm–1. 1H NMR (300 MHz, CDCl3) δ 2.75 (2H, m), 2.95 (2H, t, J = 7.5 Hz), 4.24 (1H, t, J = 7.5 Hz), 6.79 (1H, d, J = 8.1 Hz), 6.88 (2H, d, J = 8.0 Hz), 7.05 (2H, m), 7.20 (5H, m), 7.34 (2H, m), 7.66 (1H, d, J = 7.7 Hz), 7.69 (2H, d, J = 7.8 Hz), 7.86 (3H, br) ppm. 13C NMR (75 MHz, CDCl3) δ 24.3, 35.1, 36.4, 110.5, 111.6, 117.2, 119.7, 121.5, 121.9, 122.1, 122.5, 122.7, 123.1, 123.3, 125.2, 127.5, 128.0, 136.8, 137.0 ppm. Results and Discussion In the initial experiments, indole 1 and crotonaldehyde 2 were used as model substrates to evaluate suitable reaction conditions for the preparation of tris-indole 3a (Scheme 1). A wide range of Brönsted and Lewis acids were examined in room temperature. The Lewis acids such as MgBr2.Et2O, phenyl phosphonic acid and phenyl phosphonic acid/SiO2 promoted the reaction in 35%–50% yields. (Table 1, entries 2-4). Under the same conditions, montmorilonit K-10 gave 3a in 55% yield after 5 h (Table 1, entries 5). Nano TiO2 and NbCl5 did not provide the desired product (Table 1, entries 6, 7). KAl (SO4)2.12H2O gave 3a in 30% yield after 5 h (Table 1, entry 8). MCM41 promoted the reaction in 70% yield (Table 1, entry 9). Different mesoporous such asAl-MCM-41, Mn(pbdo)2Cl2/MCM-41 and Mn(pbdo)2Cl2/SBA15 promoted the reaction in 55%–65% yield after 5 h (Table 1, entries 10–12). However, among the catalysts studied for this reaction, H3PMo12O40 was found to be the most effective catalysts since it resulted in the highest conversion to the desired product (Table 1, entry 1). On the other hand, the

Catalysts Solvent, r.t.

1

2

3a Scheme 1. The synthesis of tris-indole 3a.

3  Zakeri, M. et al.

Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

Table 1. Effect of catalysts on the preparation of 1,1,3-triheteroaryl compounds.

Catalysts Solvent, r.t.

1

2 Entry 1 2 3 4 5 6 7 8 9 10 11 12

a

3a Catalyst

H3PMo12O40 (3 mol%) MgBr2. Et2O (10 mol%) Phenyl phosphonic acid (10 mol%) Phenyl phosphonic acid/SiO2 (0.1 g) Montmorilonit K10 (0.05 g) Nano-TiO2 (10 mol%) NbCl5 (10 mol%) KAl (SO4)2.12 H2O (10 mol%) MCM-41(0.05 g) Al-MCM-41 (0.05 g) Mn(pbdo)2Cl2/MCM-41(0.05 g) Mn(pbdo)2Cl2/SBA-15 (0.05 g)

Time (h)

Yielda (%)

2 5 5 5 5 No reaction No reaction 5 3 5 5 5

80 50 40 35 55 No reaction No reaction 30 70 65 55 55

The yields refer to isolated products

advantages of H3PMo12O40 as a solid acid catalyst with high activity may include large number of balanced protons and strong bronsted acidity compared with the acidity of usual mineral acids.

and ethanol/water. The results are summarized in Table 2. In comparison with other solvents, EtOH/ H2O under room temperature conditions was found to give the best result (Table 2, entry 5).

To adjust the amount of catalyst, different experiments were accomplished and it was found that by increasing the amount of catalyst from 1 to 2 and 3 mol%, the yields increased from 70 to 74 and 80%, respectively. The use of 3 mol% H3PMo12O40 was found to be sufficient to push this reaction forward and more catalyst amount (5 mol%) did not improve the yields.

As shown in Table 3, to establish the generality of the method, a wide variety of suitable substrates were employed. Various α,β-enals and indols were used and the reactions afforded the corresponding products under the optimized reaction conditions. The reaction afforded a library of 1,1,3-triheteroaryl compounds in good yields.

In another experiment, the reaction of indole 1 and crotonaldehyde 2 in the presence of 3 mol% H3PMo12O40 was used as the model system to evaluate the suitability of the media for this reaction. The reaction was performed in various solvents of water, ethanol, chloroform, acetonitrile

The plausible mechanism for synthesis of 1,1,3-triheteroaryl compounds is shown in Scheme 2. The 1,4- and 1,2-additions of indole to α,ß-unsaturated aldehyde took place sequentially to give intermediate A in the presence of H3PMo12O40 (Scheme 3). It was thought that H3PMo12O40 promoted the reaction by increasing

Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

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Table 2. Optimization of reaction conditions.

H3PMo12O40 (3 mol%) Solvent, r.t.

1

2 Entry 1 2 3 4 5

a

Solvent H2O EtOH CHCl3 CH3CN EtOH/H2O

3a Temperature (°C)

Time (min)

Yield (%)a

r.t. r.t. r.t. r.t. r.t.

150 120 120 120 120

60 72 50 55 80

Isolated yield

Scheme 2. The plausible mechanism for synthesis of 1,1,3-triheteroaryl compounds.

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Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

Table 3. Synthesis of 1,1,3-triheteroaryl derivatives 3 in presence of H3PMo12O40. Entry

Indole derivatives

α,β-unsaturated aldehydes

Products

m.p. (°C)

Yielda (%)

Found

Lit.

70

185–187

180–185 [10]

65

138–141

136–141 [11]

65

158–160

158–159 [7]

75

179–180

184–186 [11]

65

177–179

178–179 [10]

60

174–175

170–174 [11]

70

162–165

158–160 [8]

60

167–170

169–171 [11]

3a

3b

3c

3d

3e

3f

3g

3h

a

The yields refer to isolated products.

Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

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1

3a

2

1st run 2nd run 4th run

80% 78% 76%

H3PMo12O40 (3 mol%) EtOH/H2O, r.t.

4th run

72% Scheme 3. Recycling of the H3PMo12O40 in the synthesis of 3a.

the electrophilic character of the enal. The dehydration of A gave another intermediate B which was further activated by Bronsted acid H3PMo12O40 and served as an electrophile to react with the third molecule of indole, affording intermediate C. The corresponding 1,1,3-triindoleincorporated product was subsequently formed from intermediate C. The recovery and reusability of the catalyst, which is very important for industrial purposes and is highly recommended for green processes, was also explored in the model reaction. To test the reusability of the catalyst, after completing the reaction, the products were removed by filtration. The catalyst was recovered by evaporation of solvent and washed with diethyl ether. Subsequently, the catalyst was dried in a vacuum at 60°C–70 °C. The results showed that the heteropoly acid could be used at least four times with about 10% reduction in the yield of the reaction (Scheme 3). Conclusions We have successfully developed a green and mild procedure for the synthesis of 1,1,3-triheteroaryl compounds in the presence of a catalytic amount of H3PMo12O40 in EtOH/H2O. The non-toxicity

of the catalyst and solvent, simple experimental procedure, heterogeneous and recyclability of the catalyst and good yields of the products are advantages. This catalyst can be reused for four times without significant loss in the catalytic activity and change of chemical structure. Therefore, it is hoped that these eco-efficient and green protocols are of great value for both synthetic and medicinal chemistry for academic research and practical applications. Acknowledgement Authors wish to acknowledge the financial support from Research University fund (Vot. # 05H16) for Universiti Teknologi Malaysia from the Malaysian Ministry of Higher Education (MOHE). References 1. Lee, C-H, Yao, C.F., Huang, S.M., Ko, S., Tan, Y.H., Lee-Chen, G.J. and Wang, Y.C. (2008) Novel 2-step synthetic indole compound 1,1,3-tri(3indolyl)cyclohexane inhibits cancer cell growth in lung cancer cells and xenograft models, Cancer 113, 815–25. 2. Kumar, A., Kumar, A., Saxena, K.A. and Shanker, K. (1988) 1-Acetyl-5-aryl-3-[(substituted indole3-yl-methylene-amino) phenyl]-4-pyrazolines as anti-inflammatory agent, Pharmazie, 43, 45–46.

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Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

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Keggin Heteropoly Acid as a Green and Recyclable Catalyst for One-pot Synthesis of 1,1,3-Triheteroaryl Compounds

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